A SCR (Selective Catalytic Reduction) catalyst is a catalyst which purifies nitrogen oxides (NOx) by reducing the nitrogen oxides with ammonia (NH3). Especially, the SCR catalyst has started to be used for a diesel engine which emits a comparatively large amount of nitrogen oxides. The SCR catalyst is referred to as “a NOx selective reduction catalyst” or as “an adding ammonia type NOx catalyst”.
Reducing nitrogen oxides by the SCR catalyst requires to provide the SCR catalyst with ammonia which serves as a reduction agent for nitrogen oxides. A conventional apparatus provides the SCR catalyst with ammonia itself, or it supplies a water (urea-water, urea aqueous solution) containing urea (CO(NH2)2═H2N—CO—NH2) to an upstream of the SCR catalyst, instead. The urea changes into ammonia and carbon dioxides on hydrolysis. The ammonia obtained by the hydrolysis reduces the nitrogen oxides in the SCR catalyst. The system where the urea-water is supplied to an upstream of the SCR catalyst is also referred to as “Urea SCR system”. The Ministry of Land, Infrastructure, Transport and Tourism of Japan reports that, on its web page dated December 18 of Heisei 15, “a Urea SCR system study group” has been established. Hereinafter, the ammonia or the urea-water, which is added or supplied as described before, is referred to as “an additive agent” or as “a reduction agent”.
Meanwhile, when an excessive amount of ammonia is supplied to the SCR catalyst directly, or when an excessive amount of ammonia is supplied to the SCR catalyst by supplying an excessive amount of urea-water in the Urea SCR system, ammonia which can not react with nitrogen oxides in the SCR catalyst is discharged from the SCR catalyst. That is, an ammonia slip occurs. Occurring the ammonia slip means that the additive agent is consumed wastefully. On the other hand, when the supplied additive agent is insufficient for nitrogen oxides, nitrogen oxides are discharged from the SCR catalyst. Therefore, it is important to control an amount of the supplied additive agent (an amount of ammonia or urea-water) appropriately in such a system using the SCR catalyst.
Accordingly, one of conventional apparatuses comprises an upstream NOx sensor and a downstream NOx sensor, disposed at a position upstream of and at a position downstream of the SCR catalyst, respectively. The conventional apparatus controls “an amount of urea-water to be supplied” based on a concentration of nitrogen oxides detected by those sensors (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2005-127256).